Stacked complementary field effect transistors

ABSTRACT

A complementary field effect transistor (CFET) structure including a first transistor disposed above a second transistor, and a first source/drain region of the first transistor disposed above a second source/drain region of the second transistor, wherein the second source/drain region comprises a recessed notch beneath the first source/drain region.

BACKGROUND

The disclosure relates generally to stacked complementary field effect transistors (CFET). The disclosure relates particularly to stacked CFET having a recessed notch formed in the epitaxially grown source-drain regions of the lower FET of the stack, where the notch is disposed beneath an isolation layer and the source-drain region of the upper FET of the stack.

Integrated circuit (IC) chips are formed on semiconductor wafers at increasingly smaller scale. In current technology nodes, transistor devices are constructed as three-dimensional (3D) field effect transistor (FET) structures. However, chipmakers face a myriad of challenges at 5 nm, 3 nm and beyond. Currently, chip scaling continues to slow as process complexities and costs escalate at each node.

Complex gate-all-around technology includes complementary FET (CFET) where nFET and pFET nanowires/nanosheets are vertically stacked on top of each other.

SUMMARY

The following presents a summary to provide a basic understanding of one or more embodiments of the disclosure. This summary is not intended to identify key or critical elements or delineate any scope of the particular embodiments or any scope of the claims. Its sole purpose is to present concepts in a simplified form as a prelude to the more detailed description that is presented later.

A complementary field effect transistor (CFET) structure including a first transistor disposed above a second transistor, and a first source/drain region of the first transistor disposed above a second source/drain region of the second transistor, wherein the second source/drain region comprises a recessed notch beneath the first source/drain region.

A complementary field effect transistor (CFET) device formed by forming stacked sets of field effect transistor channel elements, forming bottom source/drain regions for a first transistor, forming a recessed notch in an upper surface of the bottom source/drain region, forming an isolation layer in the notch and above the bottom source/drain region, and forming a second source/drain regions above the notch.

BRIEF DESCRIPTION OF THE DRAWINGS

Through the more detailed description of some embodiments of the present disclosure in the accompanying drawings, the above and other objects, features and advantages of the present disclosure will become more apparent, wherein the same reference generally refers to the same components in the embodiments of the present disclosure.

FIG. 1A provides a schematic plan view of device, according to an embodiment of the invention. The figure illustrates the location of the section lines associated with the respective views of FIGS. 1B-17 .

FIG. 1B provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates a formed and patterned stack of epitaxially grown nanosheet layers.

FIG. 2 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates dummy gate structures formed above the stack of nanosheet layers.

FIG. 3 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after the removal of a sacrificial layer between upper and lower nanosheet elements.

FIG. 4 provides a cross-sectional view, of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after the addition of a dielectric spacer between nanosheet devices and the addition of protective dielectric sidewalls to the dummy gate structures.

FIG. 5 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after the selective removal of excess dielectric material from vertical surfaces.

FIG. 6 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after recessing the individual CFET stacks and the formation of inner CFET stack spacers.

FIG. 7 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after the epitaxial formation of source/drain regions for the lower nanosheet device.

FIG. 8 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after the deposition of a protective sacrificial dielectric around portions of the source/drain regions of the lower nanosheet device.

FIG. 9 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after the recessing of notches in the upper surfaces of lower nanosheet device source/drain regions.

FIG. 10 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after removal of the sacrificial dielectric along one side of the lower device S/D regions.

FIG. 11 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after deposition of a protective dielectric surrounding and above the S/D regions of the lower nanosheet devices.

FIG. 12 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after the epitaxial growth of upper device source/drain regions.

FIG. 13 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after encapsulation of the upper device source/drain regions.

FIG. 14 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after formation of high-k metal gate structures for the upper and lower devices.

FIG. 15 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after formation of upper and lower device source/drain region contact vias.

FIG. 16 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after removal of sacrificial layer material adjacent to the lower device source/drain regions.

FIG. 17 provides a cross-sectional view of a step in the fabrication of a semiconductor device, according to an embodiment of the invention. The figure illustrates the device after formation of contact for individual S/D regions.

FIG. 18 provides a flowchart depicting operational steps for forming semiconductor device, according to an embodiment of the invention.

DETAILED DESCRIPTION

Some embodiments will be described in more detail with reference to the accompanying drawings, in which the embodiments of the present disclosure have been illustrated. However, the present disclosure can be implemented in various manners, and thus should not be construed to be limited to the embodiments disclosed herein.

It is to be understood that aspects of the present invention will be described in terms of a given illustrative architecture; however, other architectures, structures, substrate materials, process features and steps can be varied within the scope of aspects of the present invention.

It will also be understood that when an element such as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements can also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements can be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

The present embodiments can include a design for an integrated circuit chip, which can be created in a graphical computer programming language and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer can transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein can be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher-level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

It should also be understood that material compounds will be described in terms of listed elements, e.g., SiGe. These compounds include different proportions of the elements within the compound, e.g., SiGe includes SixGel-x where x is less than or equal to 1, etc. In addition, other elements can be included in the compound and still function in accordance with the present principles. The compounds with additional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment”, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”, “and/or”, and “at least one of”, for example, in the cases of “A/B”, “A and/or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and/or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This can be extended, as readily apparent by one of ordinary skill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particular embodiments only and is not tended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, can be used herein for ease of description to describe one element's or feature's relationship to another element(s) or feature(s) as illustrated in the FIGS. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the FIGS. For example, if the device in the FIGS. is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. The device can be otherwise oriented (rotated 90 degrees or at other orientations and the spatially relative descriptors used herein can be interpreted accordingly. In addition, be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers cat also be present.

It will be understood that, although the terms first, second, etc. can be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the scope of the present concept.

Deposition processes for the metal liners and sacrificial materials include, e.g., chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or gas cluster ion beam (GCIB) deposition. CVD is a deposition process in which a deposited species is formed as a result of chemical reaction between gaseous reactants at greater than room temperature (e.g., from about 25° C. about 900° C.). The solid product of the reaction is deposited on the surface on which a film, coating, or layer of the solid product is to be formed. Variations of CVD processes include, but are not limited to, Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD), Plasma Enhanced CVD (PECVD), and Metal-Organic CVD (MOCVD) and combinations thereof may also be employed. In alternative embodiments that use PVD, a sputtering apparatus may include direct-current diode systems, radio frequency sputtering, magnetron sputtering, or ionized metal plasma sputtering. In alternative embodiments that use ALD, chemical precursors react with the surface of a material one at a time to deposit a thin film on the surface. In alternative embodiments that use GCIB deposition, a high-pressure gas is allowed to expand in a vacuum, subsequently condensing into clusters. The clusters can be ionized and directed onto a surface, providing a highly anisotropic deposition.

One of the processing complexities of CFETs that needs to be addressed at nodes beyond 5 nm is independently growing the nFET and pFET source/drain epitaxy while maintaining vertical integration and electrical disconnection. Using a conventional nanowire/nanosheet source/drain epitaxy process for CFETs forms superposed n-doped epitaxy and p-doped epitaxy, making it challenging to form independent upper and lower device source/drain regions having sufficient electrical isolation to prevent device shorting or other device reliability issues due to the close proximity of the upper and lower device source/drain (S/D) regions. Disclosed embodiments provide CFET structures and a method of forming CFETs with stacked S/D regions that maintain vertical integration and electrical disconnection of the nFET and pFET source/drain epitaxy.

Disclosed embodiments provide CFET structures including stacked and electrically isolated source/drain regions for CFET, where the physical separation of the upper and lower S/D regions is enhanced by recessing portions of the lower S/D regions directly below the upper S/D regions. This provides additional spacing for depositing isolating dielectric materials between the upper and lower device S/D regions. Disclosed embodiments are described through examples embodying nanosheet field effect transistors. The invention should not be considered limited in any manner to the nanosheet structures of the examples.

Reference is now made to the figures. The figures provide schematic cross-sectional illustration of semiconductor devices at intermediate stages of fabrication, according to one or more embodiments of the invention. The figures provide a front cross-section (X) and side cross-section (Y), taken along section lines X and Y, of the plan view of FIG. 1A. The figures provide schematic representations of the devices of the invention and are not to be considered accurate or limiting with regards to device element scale.

FIG. 1A provides a schematic plan view of a device 100, according to an embodiment of the invention. As shown in the Figure, gate structures 12, are disposed perpendicular to nanosheet stack 14. Section lines X and Y indicate the viewpoints of the respective views of FIGS. 1B-17 .

FIG. 1B provides a schematic view of a device 100 according to an embodiment of the invention following the deposition, patterning, and selective removal of material leaving a stack of layers for the formation of nanosheet CFET devices. In an embodiment, the stack includes alternating layers of epitaxially grown silicon germanium (SiGe) 140, 150, and silicon 130. Other materials having similar properties may be used in place of the SiGe and Si.

The terms “epitaxially growing and/or depositing” and “epitaxially grown and/or deposited” mean the growth of a semiconductor material on a deposition surface of a semiconductor material, in which the semiconductor material being grown has the same crystalline characteristics as the semiconductor material of the deposition surface. In an epitaxial deposition process, the chemical reactants provided by the source gases are controlled and the system parameters are set so that the depositing atoms arrive at the deposition surface of the semiconductor substrate with sufficient energy to move around on the surface and orient themselves to the crystal arrangement of the atoms of the deposition surface. Therefore, an epitaxial semiconductor material has the same crystalline characteristics as the deposition surface on which it is formed.

The nanosheet stack includes a bottom-most layer of a first semiconductor material, such as SiGe and a top-most layer of a second semiconductor material, such as Si. The nanosheet stack is depicted with ten layers (three SiGe layers and two Si layers forming a lower device, two SiGe layers and two Si layers forming an upper device, and a high Ge concertation, e.g., 50%-70% Ge, SiGe layer 150, separating the upper and lower devices), however any number and combination of layers can be used so long as the layers alternate between SiGe and Si to form lower and upper devices and include a high Ge concentration SiGe layer separating the lower and upper devices. The nanosheet stack is depicted with the layers being in the form of nanosheets, however the width of any given nanosheet layer can be varied so as to result in the form of a nanowire, a nanoellipse, a nanorod, etc. SiGe layers 140, 150, can be composed of, for instance, SiGe₂₀₋₆₀, examples thereof including, but not limited to SiGe₂₀, SiGe₂₅, SiGe₃₀ . . . SiGe₆₅.

Substrate 110 can be composed of any currently known or later developed semiconductor material, which may include without limitation, silicon, germanium, silicon carbide, and those consisting essentially of one or more III-V compound semiconductors having a composition defined by the formula Al_(X1)Ga_(X2)In_(X3)As_(Y1)P_(Y2)N_(Y3)Sb_(Y4), where X1, X2, X3, Y1, Y2, Y3, and Y4 represent relative proportions, each greater than or equal to zero and X1+X2+X3+Y1+Y2+Y3+Y4=1 (1 being the total relative mole quantity). Other suitable substrates include II-VI compound semiconductors having a composition Zn_(A1)Cd_(A2)Se_(B1)Te_(B2), where A1, A2, B1, and B2 are relative proportions each greater than or equal to zero and A1+A2+B1+B2=1 (1 being a total mole quantity). An insulating layer 120 may be present on substrate 110 and, if present, is located between substrate 110 and the nanosheet stack. Insulating layer 120 can be, for example, a buried oxide layer (typically SiO₂) or a bottom dielectric isolation layer formed early in the process (typically SiN, SiBCN, SiOCN, SiOC, or any combination of low-k materials).

In an embodiment, each sacrificial semiconductor material layer 140 and 150, is composed of a first semiconductor material which differs in composition from at least an upper portion of the semiconductor substrate 110. In one embodiment, the upper portion of the semiconductor substrate 110 is composed of silicon, while each sacrificial semiconductor material layers 140 and 150 is composed of a silicon germanium alloy. In such an embodiment, the SiGe alloy that provides each sacrificial semiconductor material layer 150 has a germanium content that is greater than 50 atomic percent germanium. In one example, the SiGe alloy that provides each sacrificial semiconductor material layer 150 has a germanium content from 50 atomic percent germanium to 70 atomic percent germanium. In such an embodiment, the SiGe alloy that provides each sacrificial semiconductor material layer 140 has a germanium content that is less than 50 atomic percent germanium. In one example, the SiGe alloy that provides each sacrificial semiconductor material layer 140 has a germanium content from 20 atomic percent germanium to 40 atomic percent germanium. The first semiconductor material that provides each sacrificial semiconductor material layers 140 and 150 can be formed utilizing an epitaxial growth (or deposition process).

Each semiconductor channel material layer 130, is composed of a second semiconductor material that has a different etch rate than the first semiconductor material of the sacrificial semiconductor material layers 140 and 150 and is also resistant to Ge condensation. The second semiconductor material of each semiconductor channel material layer 130, may be the same as, or different from, the semiconductor material of at least the upper portion of the semiconductor substrate 110. The second semiconductor material can be a SiGe alloy provided that the SiGe alloy has a germanium content that is less than 50 atomic percent germanium, and that the first semiconductor material is different from the second semiconductor material.

In one example, at least the upper portion of the semiconductor substrate 110 and each semiconductor channel material layer 130 is composed of Si or a III-V compound semiconductor, while each sacrificial semiconductor material layer 140, 150 is composed of a silicon germanium alloy. The second semiconductor material of each semiconductor channel material layer 130, can be formed utilizing an epitaxial growth (or deposition process).

Following deposition of the stack of layers 130, 140, and 150, across the surface of the device die, the layers are patterned using a process such as lithographic masking, and selectively etched, yielding a pattern of device fins including stacks of upper and lower device nanosheets separated by sacrificial layers of semiconductor materials. Such stacks define the active regions of the devices.

FIG. 2 illustrates device 100 following the forming at least one dummy gate structure on the nanosheet stack. Three dummy gates are shown however any number of gates can be formed. Dummy gate structures can be formed by depositing a dummy gate material 210 over the nanosheet stack. The dummy gate material can be, for example, a thin layer of oxide, followed by polycrystalline silicon, amorphous silicon or microcrystal silicon. After that, a hardmask layer 220 is deposited over the dummy gate, followed by lithographic patterning, masking, and etching processes.

In an embodiment, hardmask 220 includes a nitride, oxide, an oxide-nitride bilayer, or another suitable material. In some embodiments, the hardmask 220 may include an oxide such as silicon oxide (SiO), a nitride such as silicon nitride (SiN), an oxynitride such as silicon oxynitride (SiON), combinations thereof, etc. In some embodiments, the hardmask 220 is a silicon nitride such as Si₃N₄.

FIG. 3 illustrates device 100 following selective removal of sacrificial layer 150 separating the upper and lower FET devices of the CFET. In an embodiment, the high Ge concentration SiGe of layer 150 may be selectively etched away without removal of sacrificial layers 140, or channel layers 130, due to the higher concentration of Ge of sacrificial layer 150 compared to sacrificial layers 140, or channel layers 130.

FIG. 4 illustrates device 100 following conformal deposition and selective etching of spacer materials to fill the void left by removal of layer 150. Spacer material 410 further forms sidewall spacers along the sidewalls of dummy gate structure 210, hardmask 220, and sidewall of the nanosheet stack at S/D epi region. In an embodiment, spacer material 410 may be the same material as hardmask 220, or may be different materials and may be comprised of any one or more of a variety of different insulative materials, such as Si₃N₄, SiBCN, SiNC, SiN, SiCO, SiO₂, SiNOC, etc. In this embodiment, after conformal deposition, selective etching, such as anisotropic reactive ion etching, removes spacer material 410 from horizontal surfaces of the intermediate stage of the device 100.

FIG. 5 illustrates device 100 after selective removal of spacer sidewalls 410 from the stack of nanosheets 130-140. In an embodiment, anisotropic etching is used to selectively remove the vertical sidewall spacers from the nanosheet stacks. In an embodiment, following partial removal of spacer material 410 from hardmask 220, formation of a protective cap 510, through deposition of a material such as SiC, or SiO₂, upon the exposed vertical surfaces of hardmask 220, provides protection against excessive removal of spacer materials 410 from the dummy gate 210 and hardmask 220.

In an embodiment, formation of protective cap 510 includes: depositing a sacrificial material, such as OPL, over the wafer, followed by etching back the OPL to reveal the top portion of the gate spacer 410, at sidewall of the hardmask 220, while spacers at sidewalls of the nanosheet stack at S/D regions are still fully covered by OPL. After that, the exposed spacer 410 is selectively removed, followed by deposition of protective cap 510, and anisotropic etching back. Removal of the sacrificial material (OPL) occurs, e.g., through an N2/H2 ash process. Finally, an anisotropic spacer etch process can be done to etch down the sidewall spacer at the nanosheet stack at S/D regions without pulling down the spacer 410 at gate sidewall which is under protective cap 510.

FIG. 6 illustrates device 100 following recessing the nanosheet stack layers 130, 140, and spacer layer 410, to form the S/D cavities for CFET devices. FIG. 6 further illustrates device 100 following formation of inner spacers between nanosheets of the respective FET devices. Portions of nanosheet stack layers 130, 140, and 410, which are not underneath gate spacers 410 and not underneath dummy gate 210, are removed. Etching generally refers to the removal of material from a substrate (or structures formed on the substrate) and is often performed with a mask in place so that material may selectively be removed from certain areas of the substrate, while leaving the material unaffected, in other areas of the substrate.

There are generally two categories of etching, (i) wet etch and (ii) dry etch. Wet etch is performed with a solvent (such as an acid) which may be chosen for its ability to selectively dissolve a given material (such as oxide), while, leaving another material (such as polysilicon) relatively intact. This ability to selectively etch given materials is fundamental to many semiconductor fabrication processes. A wet etch will generally etch a homogeneous material (e.g., oxide) isotropically, but a wet etch may also etch single-crystal materials (e.g. silicon wafers) anisotropically. Dry etch may be performed using a plasma. Plasma systems can operate in several modes by adjusting the parameters of the plasma. Ordinary plasma etching produces energetic free radicals, neutrally charged, that react at the surface of the wafer. Since neutral particles attack the wafer from all angles, this process is isotropic. Ion milling, or sputter etching, bombards the wafer with energetic ions of noble gases which approach the wafer approximately from one direction, and therefore this process is highly anisotropic. Reactive-ion etching (RIE) operates under conditions intermediate between sputter and plasma etching.

After generally etching the nanosheet stack, a selective etching of SiGe layers 140 of the nanosheet stack removes portions which are underneath gate spacers 410. Inner spacers 610 are then formed in etched portions and thus are located under gate spacers 410. Inner spacers 610 can be composed of any suitable dielectric material, for example Si₃N₄, SiBCN, SiNC, SiN, SiCO, SiO₂, SiNOC, etc. The inner spacer is formed by a conformal dielectric liner deposition followed by isotropic etching back, so dielectric liner is removed everywhere except the regions pinched-off in those under spacer cavities.

FIG. 7 illustrates device 100 following epitaxial growth of source/drain regions 710, for the lower FET device of the CFET. In an embodiment, pairs of epitaxial source/drain regions are formed on opposing sides of nanosheet stacks and dummy gate structures. In an embodiment, boron doped SiGe (SiGe:B) is epitaxially grown from exposed semiconductor surfaces (layer 130). In an embodiment, deposition of a sacrificial material, such as OPL, covers the bottom nanosheet channel 130 sidewalls. A sacrificial spacer, such as a thin SiO₂ or SiN, then covers the top nanosheet channel 130 sidewalls. The sacrificial material, such as OPL, can be removed by N2/H2 ash, followed by bottom S/D epitaxial 710 growth. After that, the sacrificial spacer can be removed from the top nanosheet channel 130 sidewalls.

In the present embodiments, the source/drain regions 710 may be doped in situ by adding one or more dopant species to the epitaxial material. The dopant used will depend on the type of FET being formed, whether p-type or n-type. As used herein, “p-type” refers to the addition of impurities to an intrinsic semiconductor that creates deficiencies of valence electrons. In a silicon-containing semiconductor, examples of p-type dopants, i.e., impurities, include but are not limited to: boron, aluminum, gallium and indium. As used herein, “n-type” refers to the addition of impurities that contributes free electrons to an intrinsic semiconductor. In a silicon containing substrate, examples of n-type dopants, i.e., impurities, include but are not limited to antimony, arsenic and phosphorous. In an embodiment, the upper S/D regions of the device comprise n-type material and the lower regions comprise p-type materials. In an embodiment, the upper S/D regions comprise p-type materials and the lower regions comprise n-type materials.

FIG. 8 illustrates device 100 following deposition, and recess of a sacrificial spacer layer 810, such as TiO_(x), (TiO₂) around lower source/drains 710. Layer 810 protects lower S/D regions 710 during the etching of the separating notch in the source/drain regions 710. This can be achieved by a conformal deposition of sacrificial spacer 810 to pinch-off the gate-to-gate space or just overfill the sacrificial spacer material then followed by a CMP. After that, a recess process is applied to the sacrificial spacer layer 810 such that top portion of the S/D epi 710 is exposed.

FIG. 9 illustrates device 100 following selective etching of exposed portions of bottom source/drain regions 710, forming separation notched between the lower S/D regions 710, and the eventual corresponding upper S/D regions.

FIG. 10 illustrates device 100 following selective masking of sacrificial layer 810 with OPL 1010 and the subsequent removal of sacrificial material 810 disposed to the left of lower S/D region 710 in cross-sectional view Y. OPL 1010 is subsequently removed.

FIG. 11 illustrates device 100 following deposition, CMP and recess of isolating layer 1110 above and around lower S/D region 710 and above sacrificial layer 810. In an embodiment, isolation layer 1110 constitutes a material such as SiO2, SiN, SiOC, and combination of these.

FIG. 12 illustrates device 100 following epitaxial growth of pairs of upper device S/D regions 1210. In an embodiment, epitaxial growth of phosphorous doped Si (Si:P) provides S/D regions for nFET devices of the CFET. S/D regions 1210 contact nanosheet layers 130 of the upper FET device.

The disclosed example provides for the fabrication of a CFET device having an upper nFET and a lower pFET. In an embodiment, the CFET includes an upper pFET and a lower nFET. In this embodiment, the appropriate doping of the upper and lower S/D regions results in the desired pattern of nFET and pFET for the CFET device.

FIG. 13 illustrates device 100 following deposition and CMP of an interlayer dielectric (ILD) material 1310, around and above the upper S/D epitaxy and the dummy gates and gate spacers 410. The Figure illustrates the device after CMP removal of protective caps 510, and hardmasks 220 from the dummy gate structures 210, exposing the upper surfaces of dummy gate 210 materials. In an embodiment, ILD 1310 constitutes a material such as SiO2, SiN, SiOC, and combination of these.

FIG. 14 illustrates device 100 following the removal of dummy gate 210, sacrificial SiGe 140, and formation of the high-k metal gate (HKMG) stack 1410, and a protective gate dielectric cap 1420. As shown in the Figure, a replacement metal gate structure has been formed in the void space created by removal of the dummy gate 210, and sacrificial SiGe layers 140. Gate structure 1410 includes gate dielectric and gate metal layers (not shown). The gate dielectric is generally a thin film and can be silicon oxide, silicon nitride, silicon oxynitride, boron nitride, SiOCN, SiBCN, SiOC, SiCN, high-k materials, or any combination of these materials. Examples of high-k materials include but are not limited to metal oxides such as hafnium oxide, hafnium silicon oxide, hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. The high-k materials may further include dopants such as lanthanum, aluminum, magnesium. Gate dielectric can be deposited by CVD, ALD, or any other suitable technique. Metal gate can include any known metal gate material known to one skilled in the art, e.g., TiN, TiAl, TiC, TiAlC, tantalum (Ta) and tantalum nitride (TaN), W, Ru, Co, Al. Metal gate may be formed via known deposition techniques, such as atomic layer deposition, chemical vapor deposition, or physical vapor deposition. It should be appreciated that a chemical mechanical planarization (CMP) process can be applied to the top surface.

In an embodiment, the replacement metal gate includes work-function metal (WFM) layers, (e.g., titanium nitride, titanium aluminum nitride, titanium aluminum carbide, titanium aluminum carbon nitride, and tantalum nitride) and other appropriate metals and conducting metal layers (e.g., tungsten, cobalt, tantalum, aluminum, ruthenium, copper, metal carbides, and metal nitrides). After formation and CMP of the HKMG 1410, the HKMG 1410 can be optionally recessed followed by a deposition and CMP of a gate dielectric material 1420 completes the replacement metal gate fabrication stage for the device.

FIG. 15 illustrates device 100 following the formation of contact vias 1510 from an upper ILD surface to an upper surface of upper device S/D regions 910 and the upper surface of sacrificial material layer 810. The Figure further illustrates the formation of a common contact via for the upper and lower S/D regions on the left side of cross-section X of the Figure.

FIG. 16 illustrates device 100 following removal of the sacrificial material 810 from its position adjacent to lower device S/D regions 710. As shown in cross-section Y of the Figure, the sacrificial semiconductor material 810, has been removed, exposing lower device S/D regions 710.

FIG. 17 illustrates device 100 following the deposition of a metal S/D contact 1710 in the S/D regions contact vias. In an embodiment, deposition of silicide liner such as Ti, Ni, Co, NiPt, followed by adhesion metal liner, such as a thin layer of TiN, followed by conductive metal such as Cu, Ag, Au, W, Co, Ru, or combinations thereof, forms the contact. The geometry of the contact reduces the contact resistance by increasing the silicide surface area between the contact and S/D epi with the S/D regions.

FIG. 18 depicts a fabrication process flowchart 1800, according to an embodiment of the invention. As shown in flowchart 1800, at block 1810, nanosheet sets for the CFET devices are formed. Stacks of alternating nanosheet layers of differing semiconductor materials are epitaxially grown upon an underlying substrate, or upon an insulating layer disposed upon a substrate. The stacks include sacrificial layers and channel layers. The channel layers form the nanosheets of the upper and lower FETs of the CFET. The layers are patterned and etched to form fins upon the underlying substrate. Dummy gate structures including sidewall spacers are added atop and along the fins. The nanosheet layers are recessed to align with the dummy gate spacers and inner spacers between nanosheet channel layers are formed to isolate the gate from the S/D regions of the devices.

At block 1820, S/D regions for the lower device are epitaxially grown upon the device in contact with the nanosheet channel layers. The S/D regions are patterned and etched back to form the final lower S/D regions in contact with the lower FET nanosheet channel layers. Prior to growing the lower S/D regions, the upper device semiconductor channels are shielded with a thin layer of sacrificial protective material.

At block 1830, a recess is formed in the upper surface of the lower device S/D regions. These notches provide additional physical separation between S/D regions of the upper and lower devices and reduce the likelihood of short circuits between the respective S/D regions. A sacrificial layer of material is deposited around the lower S/D regions, leaving a portion of the S/D regions exposed. The exposed portions are then selectively etched away and further recessed, creating the separating notches.

At block 1840, an isolation layer is formed above the lower S/D region. The isolation layer is formed around and above the lower device S/D regions and the remaining sacrificial material layer. The isolation layer fills the recessed notches of the lower device S/D regions and covers the remaining sacrificial layer material.

At block 1850, upper device S/D regions are formed from the upper device semiconductor nanosheet channels and upon the isolation layer upper surface.

At block 1860, the HKMG structure is formed as a replacement for the dummy gate structure. The dummy gate is removed, and a high-k layer is deposited followed by deposition of a work function metal and completed by deposition of a sacrificial protective cap upon the HKMG structure.

At block 1870, independent source/drain contacts are formed for CFET device. Vias are etched through protective dielectric material layers to expose the upper S/D regions and the sacrificial material disposed around the lower S/D regions of a first side of the devices. A first via exposes the upper S/D region on one side of the devices. A second via exposes the material adjacent to the lower S/D region on the same side of the devices. The sacrificial material is then removed exposing the lower S/D regions. A third common via exposes the upper and lower S/D regions of the other side of the devices. Contact metal is then disposed in the vias providing electrically independent contacts to each of the upper and lower S/D regions of the devices.

The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and device fabrication steps according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more fabrication steps for manufacturing the specified device(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved.

References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

The descriptions of the various embodiments of the present invention have been presented for purposes of illustration but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. 

What is claimed is:
 1. A CFET structure (complementary field effect transistor) comprising: a first transistor disposed above a second transistor; and a first source/drain region of the first transistor disposed above a second source/drain region of the second transistor, wherein the second source/drain region comprises a recessed notch beneath the first source/drain region.
 2. The CFET structure according to claim 1, further comprising a first dielectric material disposed in the notch and in contact with a bottom surface of the first source-drain region.
 3. The CFET structure according to claim 1, further comprising a metal contact disposed in contact with the first source/drain region.
 4. The CFET structure according to claim 1, further comprising a second metal contact disposed in contact with the second source/drain region.
 5. The CFET structure according to claim 1, further comprising a metal gate structure disposed adjacent to the first source/drain region and the second source drain region.
 6. The CFET structure according to claim 1, wherein the first transistor comprises an nFET.
 7. The CFET structure according to claim 1, wherein the first transistor comprises a pFET.
 8. A CFET structure (complementary field effect transistor) comprising: a first transistor disposed above a second transistor; and a pair of first source/drain regions of the first transistor disposed above a pair of second source/drain regions of the second transistor, wherein each of the pair of second source/drain regions comprises a recessed notch beneath a corresponding first source-drain region.
 9. The CFET structure according to claim 8, further comprising a first dielectric material disposed between the first transistor and the second transistor, wherein the first dielectric material is disposed in the notch.
 10. The CFET structure according to claim 8, further comprising a first metal contact disposed in contact with one of the first source/drain regions.
 11. The CFET structure according to claim 8, further comprising a second metal contact disposed in contact with one of the second source/drain regions.
 12. The CFET structure according to claim 8, further comprising a metal gate structure disposed between the pair of first source/drain regions and the pair of second source drain regions.
 13. The CFET structure according to claim 8, wherein the first transistor comprises an nFET.
 14. The CFET structure according to claim 8, wherein the first transistor comprises a pFET.
 15. A method of fabricating a semiconductor device, the method comprising: forming stacked sets of field effect transistor channel elements; forming first source/drain regions for a first transistor; forming a recessed notch in an upper surface of the first source/drain region; forming an isolation layer in the notch and above the first source/drain regions; and forming second source/drain regions for a second transistor above the notch.
 16. The method of fabricating a semiconductor device according to claim 15, further comprising forming a complementary field effect transistor stack including the first transistor and the second transistor.
 17. The method of fabricating a semiconductor device according to claim 15, further comprising forming a first source/drain contact connecting to the first source/drain region.
 18. The method of fabricating a semiconductor device according to claim 15, further comprising forming a second source/drain contact connecting to a source drain region of the second transistor.
 19. The method of fabricating a semiconductor device according to claim 15, wherein the first transistor comprises an nFET.
 20. The method of fabricating a semiconductor device according to claim 15, further comprising forming a high-k metal gate structure in contact with at least one of the source/drain regions. 